The present invention relates to an exhaust gas purifying catalyst, and more particularly to an exhaust gas purifying catalyst that is capable of executing both an NOx absorbing function and a three-way function.
A lean combustion type engine such as a lean burn engine and a direct injection type engine is operated at a leaner air-fuel ratio than a stoichiometrical air-fuel ratio in a predetermined operating range in order to improve the fuel economy characteristics and the exhaust gas characteristics. However a three-way catalyst cannot sufficiently purify NOx in the exhaust gases while the engine is operated at the lean air-fuel ratio. Accordingly, it is known that while the engine is operated at lean air-fuel ratio, nitrogen oxide (NOx) is absorbed from exhaust gases in oxide atmosphere and the absorbed NOx is reduced to nitrogen (N2) in reduced atmosphere so as to reduce the output of NOx into the air.
To provide a catalyst with an NOx absorbing function, alkali metals and alkali earth metal, which can be nitrate such as potassium (K) and barium (Ba), are added as an absorbent agent to a catalyst so that the catalyst can function as an NOx absorbent catalyst as disclosed in Japanese laid-open Patent Publication No. 9-85093.
FIG. 8 shows an NOx purification rate (a value which is measured fifteen seconds after the air-fuel ratio changes to the lean air-fuel ratio) and the total HC (THC) purification rate (a value which is measured at an air-fuel ratio of 30). As is clear from FIG. 8, the NOx absorbing function and the three-way function (the oxidization/reducing function) have a tradeoff relationship. FIG. 9 shows the function expressing a relationship between the NOx purification rate and the THC purification rate after a heat endurance test and the amount of absorbent agent added to a catalyst. During the heat endurance test, a part of the absorbent agent dissipates due to vaporization at a high temperature and the consumption resulting from reaction of the absorbent agent and a cordierite carrier. For this reason, the NOx purification rate after the heat endurance test is lower than that before the heat endurance test, although the THC purification rate after the heat endurance test is higher than that before the heat endurance test. Thus, the NOx purification rate and the THC purification rate have a tradeoff relationship as is the case with FIG. 9. It is therefore difficult for a single catalyst to execute both the NOx absorbing function and the three-way function in a desired manner. Since operation at only the lean air-fuel ratio cannot satisfy a driver""s output requirement, a catalyst system needs to execute the three-way function of purifying CO and HC generated during operation at both the stoichiometrical air-fuel ratio and the rich air-fuel ratio. The three-way function is also required for reducing NOx, which is absorbed during the operation at the lean air-fuel ratio. Thus, in order to assure the three-way function, it is ordinarily necessary to intentionally inhibit the NOx absorbing function by restricting the amount of added absorbent agent or forming the absorbent agent by a material having a low absorbent capability. Therefore, a single catalyst cannot satisfactorily execute both the NOx absorbing function and the three-way function. To execute both functions in a desired manner, it is necessary to separately provide an NOx catalyst, which is specialized in absorbing NOx (to which a sufficient amount of absorbent agent is added), and a normal three-way catalyst.
It is therefore an object of the present invention to provide an exhaust gas purifying catalyst that is configured as a single catalyst and is able to execute both an NOx absorbing function and a three-way function in a desired manner.
To attain the above object, the present invention provides an exhaust gas purifying catalyst which includes a carrier, an NOx absorbent catalyst layer and a three-way catalyst layer, and in which at least one absorbent agent selected from a group of alkali metals and alkali earth metals is added to the NOx absorbent catalyst layer, the exhaust gas purifying catalyst wherein: an effect inhibiting material for inhibiting an effect of the absorbent agent on the three-way catalyst is added to at least one of the NOx absorbent catalyst layer and the three-way catalyst. The absorbent agent, which reaches the three-way catalyst, disturbs the CO and HC absorption onto the noble metal of the three-way catalyst due to its electron donating property and moves on a noble metal active surface to cover the surface. The effect inhibiting material inhibits such adverse effects. Consequently, the NOx absorbent catalyst layer and the three-way catalyst layer can be formed on one carrier to thereby form a single catalyst that is able to execute the NOx absorbing function and the three-way function by the NOx absorbent catalyst layer and the three-way catalyst layer, respectively.